The present invention relates to a magnetic recording medium for encoders, and more particularly relates to a magnetic recording medium advantageously used for a high precision encoder which is used as a mechatronics sensor for detection of displacement of an arm of a robot.
The most typical conventional magnetic recording medium for this application is made up of a substrate and a thin magnetic layer deposited on the substrate and the magnetic layer is made of ferrite, Fe--Cr--Co alloy or rare earth bonded magnet.
When ferrite or Fe--Cr--Co is used, their relatively low coercive force (Hc) or low (BH) max allows generation of low intensity outputs only. As a consequence, the distance between a magnetic recording medium and an associated magnetic sensor must be as small as one half of the magnetization pitch of the information stored in the recording medium. In order to successfully provide such a small detective distance, the assembly of the encoder has to be carried out with an extremely high precision. In addition, it is almost unfeasible in practice to make the detective distance so small as required from the above-described view point of material. For these reasons, there is no other choice but to adopt a relatively large magnetization pitch of the information to be stored in the recording medium. Such an enlarged magnetization pitch, however, inevitably prevents detection of displacement of a mobile object with high precision.
Use of rare earth bonded magnet removes the problem of the small detective distance thanks to its relatively high coercive force. Despite that merit, use of this material is accompanied with a great disadvantage resulted from particle size distribution of magnet particles dispersed in the binder of the magnet. That is, uneven particle size distribution tends to cause undesirable disorder in lines of magnetic force generated by the magnetic recording medium. Such uneven distribution inevitably results in low precision in detection of displacement.